There's a huge difference between wearing a t-shirt, which makes the information available only to people who see you wearing that t-shirt, and becoming a fan of a brand on Facebook, which makes the information available to who knows what Facebook affiliate.

In one scenario, you have full control over when you signal that information and when you stop. In the other, once you send out the signal... congratulations, you're now in several databases and have opened yourself to all sorts of soliciting.